The present invention relates to a fluidized bed unit and a method for operating same in which material removed from the bed is cooled before being passed to external equipment.
The use of fluidized beds has been recognized as an attractive means of generating heat. In these arrangements, a bed of particulate material is provided which normally consists of a mixture of inert material, a fossil fuel such as coal, and an adsorbent for the sulfur formed during the combustion of the fuel. Air is passed through the bed to fluidize the bed and to promote the combustion of the fuel. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, a reactor, or the like, the fluidized bed offers an attractive combination of high heat release, improved heat transfer to surfaces within the bed, and compact size.
During the operation of these units new adsorbent material, usually limestone, must be continuously introduced into the bed to insure the existence of an adequate supply to maintain desired sulfur emissions control, and spent material, which may include ash from combusted fuel, must be continuously withdrawn from the bed to maintain a desired bed inventory or consistency. However, the withdrawal of material from the bed presents problems. For example, the removed material is at a very high temperature, such as 1300.degree. F. to 2100.degree. F., and, since it is usually passed to external materials handling equipment such as auger blades, or other rotary devices, the latter are easily damaged due to the high temperatures and pressure differentials involved. This of course requires frequent replacement of the equipment and/or the use of expensive components which are less immune to deterioration with the high temperatures. Also the removed bed material may contain incompletely reacted fuel residues and/or environmentally unstable products such as alkali sulfides, as well as unreacted chemicals such as lime, or the like, for use in adsorbing the sulfur generated during the burning process. As a result of this, plus the fact that the bed material is at a very high temperature as discussed above, the removal of the material will result in a considerable loss of heat and unreacted bed material from the process.